The present invention relates to an electromagnetic flowmeter which measures the flow rate of a fluid to be measured, which flows through a measuring pipe and, more particularly, to an exciting method and signal processing method capable of realizing accurate flow rate measurement.
An electromagnetic flowmeter measures the flow rate of a conductive fluid to be measured, which flows through a measuring pipe, by converting the flow rate into an electrical signal by using electromagnetic induction. FIG. 25 shows the arrangement of a conventional electromagnetic flowmeter. This electromagnetic flowmeter has a measuring pipe 11, a pair of electrodes 12a and 12b, an exciting coil 13, a signal conversion unit 15, and a flow rate output unit 16. A fluid to be measured flows through the measuring pipe 11. The pair of electrodes 12a and 12b oppose each other in the measuring pipe 11 to be perpendicular to both the magnetic field applied to the fluid to be measured and an axis PAX of the measuring pipe 11 and also come into contact with the fluid to be measured. The electrodes 12a and 12b detect an electromotive force generated by the magnetic field and the flow of the fluid to be measured. The exciting coil 13 applies, to the fluid to be measured, a magnetic field perpendicular to both the measuring pipe axis PAX and an electrode axis EAX that connects the electrodes 12a and 12b. The signal conversion unit 15 detects the electromotive force between the electrodes 12a and 12b. The flow rate output unit 16 calculates the flow rate of the fluid to be measured on the basis of the interelectrode electromotive force detected by the signal conversion unit 15.
In the electromagnetic flowmeter shown in FIG. 25, a plane PLN which includes the electrodes 12a and 12b and is perpendicular to the direction of the measuring pipe axis PAX is defined as a boundary in the measuring pipe 11. At this time, symmetrical magnetic fields are applied to the fluid to be measured on both sides of the plane PLN, i.e., the boundary in the measuring pipe 11. The exciting coil 13 can be excited by a sine wave exciting method capable of high-frequency excitation or a rectangular wave exciting method which is not affected by electromagnetic induction noise.
The sine wave exciting method that uses a sine wave as an exciting current for an exciting coil is readily affected by commercial frequency noise. However, this problem can be solved by a high-frequency exciting method which uses an exciting current having a higher frequency. The high-frequency exciting method is resistant to 1/f noise such as electrochemical noise or spike noise. In addition, this method can improve the response (a characteristic which makes a flow rate signal quickly follow a change in flow rate).
However, the conventional sine wave exciting method is readily affected by in-phase component noise. An example of in-phase component noise is a shift of the amplitude of a magnetic field applied to a fluid to be measured. In the conventional electromagnetic flowmeter, when the amplitude of the exciting current supplied to the exciting coil varies (shifts) due to a fluctuation in power supply voltage, and the amplitude of the magnetic field applied to the fluid to be measured shifts, the amplitude of the interelectrode electromotive force changes, resulting in a flow rate measurement error due to the influence of shift. Such in-phase component noise cannot be removed even by the high-frequency exciting method.
To the contrary, the rectangular wave exciting method that uses a rectangular wave as an exciting current to be supplied to an exciting coil is resistant to in-phase component noise. In the rectangular wave exciting method, however, the interelectrode electromotive force is detected when a change in magnetic field becomes zero. When the exciting current has a high frequency, the detector must have high performance. Additionally, in the rectangular wave exciting method, when the exciting current has a high frequency, effects of the impedance of the exciting coil, the exciting current response, the magnetic field response, and an overcurrent loss in the core of the exciting coil or measuring pipe cannot be neglected. It is difficult to maintain rectangular wave excitation. As a result, in the rectangular wave exciting method, high-frequency excitation is difficult, and an increase in response to a change in flow rate or removal of 1/f noise cannot be realized.